1. Field of the Invention
The present invention relates to an electrostatic powder spray gun and a coating method used for decorative coating such as automotive clear top coating and the like.
2. Description of the Related Art
Attention has been paid to electrostatic powder coating as a nonpolluting coating method using no solvent from the ecological point of view. In electrostatic powder coating, a powder coating is supplied from a powder hopper to a spray gun through an injector and sprayed toward an object to be coated from a nozzle opening formed at an extreme end of the spray gun together with a conveying air flow. At this time, a high voltage is applied to a pin type electrode disposed at the extreme end of the spray gun as the object to be coated is electrically grounded and corona discharge is generated from the electrode of the spray gun toward the object to be coated. As a result, when the powder coatings sprayed from the nozzle opening pass by the vicinity of the electrode, the powder coatings are charged by the collision thereof with the ions generated by the corona discharge. The thus charged powder particles are coated on the surface of the object to be coated by being affected by the conveying air flow and electric lines of force.
Although powder coatings having an average particle size of 30-40 .mu.m usually have been used in electrostatic powder coating, since an obtained coating film has smoothness inferior to that of solvent coating, it has been attempted to obtain a decorative coating film excellent in smoothness using fine particle coatings having an average particle size of 25 .mu.m or less.
However, when the particle size of powder is reduced, the powder is condensed or is liable to be adhered to a powder hose, a spray gun and the like by being strongly affected by electrostatic force. As a result, since it is difficult to stably supply fine particle powder coatings to the spray gun, it is difficult to form a coating film having desired smoothness.
Further, since many of the ions generated by corona discharge adhere to the surface of an object to be coated as free ions, as powder particles deposit on the object to be coated, the free ions also accumulate on the surface of the object to be coated. Thus, there also arises a problem that the electrostatic potential on the surface of the object to be coated gradually increases to thereby lower the transfer efficiency of the powder particles. Further, when the field intensity in a powder particle layer increases as described above and exceeds the dielectric breakdown field intensity level of air, minute discharge takes place in the powder particle layer, that is, a so-called reverse ionization phenomenon is caused with a consequent rough surface of a coating film.